In order to increase efficiency and performance of gas turbines so as to provide increased power generation, lower emissions and improved specific fuel consumption, turbines are tasked to operate at higher temperatures and under harsher conditions. Such conditions become a challenge for cooling of certain materials.
As operating temperatures have increased, new methods of cooling alloys have been developed. For example, ceramic thermal barrier coatings (TBCs) are applied to the surfaces of components in the stream of the hot effluent gases of combustion to reduce the heat transfer rate and to provide thermal protection to the underlying metal and allow the component to withstand higher temperatures. Also, cooling holes are used to provide film cooling to improve thermal capability or protection. Concurrently, ceramic matrix composites (CMCs) have been developed as substitutes for some alloys. The CMCs provide more desirable temperature and density properties in comparison to some metals; however, they present additional challenges.
Processing laminated composite turbine airfoils, such as with CMCs, has been shown to be effectively executed using melt infiltration (MI). Problems exist, however, when the laminates become thick and/or the geometry becomes complex. These conditions create more problems for the inflow of the matrix material into the fiber laminate resulting in regions of porosity and/or silicon-rich zones, both of which can be initiation sites for crack propagation and ensuing fracture for interlaminar separation.
The problems are especially acute when processing thick sections where the final liquid silicon volumes fail to fill effectively leading to micro-porosity in the matrix. Additionally, thick sections present a thermal inertia which manifests as time-varying solidification fronts where the last-to-solidify regions sometimes do not transform to SiC and instead form undesirable features rich in free silicon (solid). Such features can lead to faster crack-growth rates through the matrix precipitating premature interlaminar failures.
A ceramic matrix composite article and a process of fabricating ceramic matrix composite articles that do not suffer from one or more of the above drawbacks would be desirable in the art.